This invention relates to a drum type line printer for printing bar codes. More particularly, the invention relates to a drum type line printer which prevents the occurrence of so-called ghost images that are generated by adjacent types during printing strokes against the bar code types by their respective hammers. The ghost images are undesirable black bar impressions that are printed where they should not be.
The need for bar code printers has recently increased due to the recent adoption of the POS (point-of-sale) system and the batch treatment system.
For example, the bar code shown in FIG. 1 of the drawings hereof is employed in such systems. This bar code is an instance of a shortened version of a bar code and comprises a plurality of parallel black bar portions and a plurality of white bar portions. The illustrated bar code comprises, in sequence from the left, a left guard bar unit, a left side character unit consisting of four characters including a flag character, a center bar unit, a right side character unit consisting of four characters including a modular check character, and a right guard bar unit. Each of the guard bar units comprises three modules. The center bar unit comprises five modules. Each character unit comprises seven modules. The module at the left end of the left side data character unit and the module at the right end of the right side data character unit are always white bar portions.
In printing such bar codes with a drum type line printer, it is common and effective to align a respective printing hammer with each guard bar row, each center bar row and each character row. The rows are arranged around the circumference of the printing drum. Thus, in order to print these bar codes in practice, a module in white line form is formed as a pseudospace between each character axially along the drum, and this spaces apart the axially arranged rows of the respective bar code types.
However, since the printing is performed such that each pseudospace between the characters in a line is the width of one module of a multi-module bar code character, the spaces between the rows of types are very small. Therefore, when one type is printed by a printing hammer, the material to be printed, such as paper or a label sheet, often contacts the adjacent types through the carbon ribbon. Thus, ghost images are often generated by the black line printing portions or modules of adjacent types. As a result, the printing information or bar code on the material to be printed may be erroneously read by an automated optical character reader.
The foregoing problem arises with the printing of all bar codes, except those in which a considerable space is left between adjacent characters in a line.